HARRISBURG — The battle over legislation to ramp up transportation spending in Pennsylvania by more than $2 billion a year is shifting to labor unions and whether they will support a provision that they say could erase gains in wages, but might be necessary to win Republican support. Labor union leaders acknowledged Monday that they are not seeing eye-to-eye on the wage provision as they lobby lawmakers ahead of a potential House vote later this week. Gov. Tom Corbett and House Speaker Sam Smith, R-Jefferson, are pressing labor leaders to accept a change to the prevailing-wage law as a way to win rank-and-file House GOP support for a measure that also would raise motorist taxes and fees. Republicans pitch the change law as a way to save taxpayer money and get more projects done. But labor leaders say it opens the door to allow contractors to roll back nonunion tradesmen’s wages on smaller projects, such as paving, guardrail work or curbs, and makes it harder for contractors that use union labor to compete for those projects. The AFL-CIO, the labor federation, and a number of other labor unions — with a few noteworthy exceptions — are fighting the change. “We want to do transportation (legislation) without cutting wages and that’s what we’re going to fight for,” said Rick Bloomingdale, president of the AFL-CIO in Pennsylvania. Prevailing wage laws require contractors on state-funded construction projects costing more than $25,000 to pay specific wages to various tradesmen. The wages are set by the state Department of Labor and Industry and are generally tied to those in the county’s organized labor contracts.